Unloading‐Based Stiffness Characterisation of Cement Pastes During the Second, Third and Fourth Day After Production

Abstract

The stiffness evolution of binder 'cement paste' is triggering the stiffness of concrete. In the engineering practice, concrete formworks are typically removed 24 h after production. This underlines that knowledge on mechanical properties of cementitious materials during the second, third and fourth day after production is of high relevance for the ongoing construction process. This provides the motivation to perform early-age stiffness characterisation on hydrating cement pastes, by means of the following three test methods. Unloading modulus is determined using a novel setup for non-destructive uniaxial compression testing including overdetermined deformation measurements. Dynamic Young's moduli are obtained from ultrasonics experiments. Isothermal differential calorimetry allows for linking the observed temporal evolution of early-age stiffness to the hydration degree of cement. Pastes with three different compositions are investigated, defined in terms of the initial water-to-cement mass ratio w/c and the initial water-to-solid (binder) mass ratio w/s. Pure cement pastes exhibit w/c = w/s = 0.50 and w/c = w/s = 0.42, respectively. A fly ashblended cement paste refers to a cement mass replacement level of 16%, and this is related to w/c = 0.50 and w/s = 0.42. Both unloading moduli and dynamic Young's moduli of all three materials increase practically linearly with increasing hydration degree, in the investigated regime of hydration degrees ranging from 40 to 60%. Fly ash does not contribute significantly to the early-age hydration of the material, i. e. it represents a quasi-inert part of the material's microstructure, exhibiting a significant stiffening effect.

Creep Experimental Study of Rocks Containing Weak Interlayer Under Multilevel Loading and Unloading Cycles

“…The AiniMR-60 nuclear magnetic resonance analyzer was used to test the porosity of the cyan sandstone specimens, and the average porosity of the cyan sandstone specimens was 4.26%. According to the method introduced in Kohlhauser and Hellmich 2013and Karte et al (2015), the primary velocity of the sample measured by the wave velocity tester is 2, 551 m • s −1 , and the secondary velocity is 1, 453 m • s −1 . The rock blocks were cut into 50 × 100 mm cylinder standard specimens in the laboratory [based on the recommendations of the International Society of Rock Mechanics on the dimensions of the test specimens (ASTM D2938, 2002].…”

Uncertainty analysis of the power law extrapolation techniques for adhesive anchors

“…However, this is not always the case and even for the case of concrete, the creep behavior is still subject to ongoing research that just recently yielded two new creep models, the RILEM recommendation B4 and the fib model code based on laboratory data and multidecade structural evidence . The main driving forces are often structural failures but also the increasing availability of long‐term experimental campaigns and new insights gained from lower‐scale investigations …”

“…[14][15][16][17] The main driving forces are often structural failures 18 but also the increasing availability of long-term experimental campaigns 14,15,19 and new insights gained from lower-scale investigations. [20][21][22] The long-term deformations of adhesive anchors are not only influenced by the visco-elastic behavior of the adhesive mortars but also by the creep behavior of concrete itself, interacting with that of the mortar. According to Bazant and Kim 23 , Findley, Lai, and Onaran 24 , Jirásek and Bažant 25 the applicability of linear visco-elasticity is limited to low stresses above which nonlinear phenomena and damage processes start to dominate the response, applying both to concrete and polymers.…”

Early-Age Evolution of Strength, Stiffness, and Non-Aging Creep of Concretes: Experimental Characterization and Correlation Analysis

“…This is because shear stresses are activated by friction in the interfaces between the specimens and the load platens. The self-equilibrated shear stresses affect the entire tested specimen [9]. They result in a confinement of the material, which increases the strength of the tested cube.…”

“…This is to be understood based on the principle of Saint Venant [10] (see also [11,12,13]). It implies that the self-equilibrated shear stresses decrease with increasing distance from the interfaces between the specimen and the load platens, such that they reach insignificant magnitudes in a distance amounting to roughly one times the characteristic in-plane dimension of the interfaces between the specimen and the load platens (see [9] for validation of this statement by means of Finite Element simulations). Thus, the central part of a cylinder with a width of 150 mm and a height of 300 mm may be considered to be virtually free of undesired shear stresses.…”